1. Field of the Invention
The present invention relates generally to methods and apparatuses for isolating two transmitters transmitting two different signals that are both driving a television antenna. More specifically, the invention relates to methods and apparatuses for transmitting two different signals from a television antenna using two orthogonal modes.
2. Description of the Related Art
Antennas for transmitting television signals typically transmit in the VHF (175 MHz to 250 MHz) frequency range or the UHF (470 MHz to 860 MHz) frequency range. In certain circumstances, it may be desirable to connect more than one transmitter to a transmitting antenna or antenna array. When this is done, it is important that energy from one transmitter that is coupled to the antenna not be coupled into another transmitter that is also coupled to the antenna. Coupling of energy from one transmitter to another transmitter would likely interfere with the operation of that transmitter and could in some cases destroy the transmitter.
In the past, it has been the practice in the United States to use individual antenna towers dedicated to a single television station and not to couple more than one signal to an antenna or antenna array. In Europe, the practice of coupling more than one signal to a single antenna or antenna array has been more common. The signals combined for transmission on a single array have generally been separated in frequency. This has enabled filters to be designed that are capable of isolating transmitters transmitting in one frequency band from other transmitters transmitting in other bands.
Recently, a need has arisen for simultaneously transmitting signals that are not separated in frequency using a single television antenna. With the advent of digital television, many television stations will, for some time at least, be required to simultaneously transmit both a digital as well as an analog version of their programming. The bandwidths that have been allocated for the separate transmissions, are, in some cases, adjacent to each other or at least very close in frequency. For example, in one scheme that is described below, adjacent 6 MHz channels are provided for simultaneous analog and digital transmission. Thus, if a television station wants to transmit both its digital signal and its analog signal using a single antenna or antenna array, then it is necessary to find a way to couple the two signals from a pair of transmitters to the single antenna or antenna array in a way that prevents the two transmitters from interfering with each other even when the two transmitters are transmitting at nearly the same frequency. A typical analog National Television Standards Committee (NTSC) television signal and a typical digital television signal are illustrated in FIG. 2. Conventional signal combining methods have not acceptably achieved the goal of separating such signals, as is detailed below.
FIG. 1A is a block diagram illustrating a star point combiner. A transmitter 100 and a transmitter 102 are both connected to a common output 103. Transmitter 100 is isolated from transmitter 102 using a highly tuned resonant circuit network 104. Transmitter 100 is connected to the left portion of highly tuned resonant circuit network 104 which is a bandpass filter for the signal from transmitter 100. Filter 104 rejects the energy from transmitter 102, but passes the energy from transmitter 100. Likewise, transmitter 102 is connected to a highly tuned resonant circuit network 105 which is a bandpass filter for the signal from transmitter 102. Filter 105 rejects the energy from transmitter 100, but passes the energy from transmitter 102.
The disadvantage of the star point combiner for the application described above is that it requires precise tuning of the bandpass filters. The absorption of energy by the filters requires an exact impedance match and the system does not work over a large bandwidth. Furthermore, the design also does not work well for two transmitters operating at nearly the same frequency.
FIG. 1B is a block diagram illustrating a commutating line combiner. The commutating line combiner includes a transformer 110 that includes two inputs for a first transmitter 111 and a second transmitter 112. The combined output of the two transmitters is obtained at output 116. The commutating line transformer depends on transmission line 118, which must have a length that corresponds to one half the wavelength at the difference in frequency between the two transmitter signals. If the frequency difference is small, then the length of transmission line 118 becomes unacceptably long. Furthermore, the frequency dependence of the combiner is undesirable and prevents it from working across a large bandwidth.
FIG. 1C is a block diagram illustrating a constant impedance combiner. The constant impedance combiner includes two inputs for a first transmitter 121 and a second transmitter 122. The signals from the two transmitters are combined at a combined output 124. In order to isolate second transmitter 122 from first transmitter 121, it is necessary to provide a pair of filters 126 and 128 which filter out the frequency band of the second transmitter. An advantage of this design is that additional combiners may be cascaded so that additional transmitters may be included. The problem with the design is the requirement of the filters. When the frequency bands of the two transmitters are close together, then it is difficult to obtain a notch filter with sharp enough roll off to filter out the frequency band of the second transmitter without affecting the signal from the first transmitter. Specifically, traditional filter devices have an attenuation slope that converts the FM modulated audio subcarrier of a NTSC analog signal into unwanted AM modulated signals. This adversely affects the video signal, which is AM modulated.
FIG. 2A is a block diagram illustrating in more detail the interference problem between a typical NTSC analog signal and a typical digital television signal when the DTV channel is assigned a 6 MHz bandwidth that is adjacent to and below the 6 MHz bandwidth assigned to an NTSC signal. The NTSC signal includes a video signal 200 and an audio signal 202 occupying the 6 MHz NTSC channel. The vestigial sideband of the video signal extends beyond the lower frequency boundary of the NTSC channel into the DTV channel. A digital television signal 210 is shown occupying the DTV channel adjacent to the NTSC analog signal. The separation between the upper frequency boundary of digital television signal 210 and the video signal is about 1.25 MHz. Because the vestigial sideband is not required by most modern systems, it is possible in certain instances to create a tuned filter that effectively blocks the portion of the video signal that extends into the DTV channel. It is essential, however, that the filter not cause amplitude modulation of the NTSC video signal. The case where the DTV channel is assigned to a bandwidth that is adjacent and above the NTSC bandwidth presents a more difficult problem because there is less frequency separation, as is shown in FIG. 2B.
FIG. 2B is a block diagram illustrating in more detail the interference problem between a typical NTSC analog signal and a typical digital television signal when the DTV channel is assigned a 6 MHz bandwidth that is adjacent to and above the 6 MHz bandwidth assigned to an NTSC signal. The NTSC signal includes a video signal 200 and an audio signal 202 occupying the 6 MHz NTSC channel. A digital television signal 210 is shown occupying the DTV channel adjacent to the NTSC analog signal. The audio signal is very close to the lower edge of the DTV channel. The separation between the lower frequency boundary of digital television signal 210 and the upper frequency boundary of audio signal 202 is as little as 250 kHz. It is exceedingly difficult to design a filter that can block the audio signal without affecting the DTV signal.
As a result of the bandwidth allocation illustrated in FIG. 2, any practical filter designed to block the digital television signal from reaching the NTSC transmitter will likely degrade the audio signal and any practical filter designed to block the NTSC signal from reaching the digital transmitter will likely degrade the digital television signal.
What is needed, therefore, is a system and method for combining signals from two television transmitters on a common antenna or antenna array that does not rely on filtering the signals. Additionally, it is preferable that the antenna array be designed so that the signals produced by the signal combiner generate a desirable omniazimuthal pattern when input to the array. Thus, a signal combiner, antenna feeding scheme and antenna array that together produce omniazimuthal patterns while isolating the inputs across a large bandwidth would be desirable.